Environment-friendly degradable packaging bag and preparation method thereof

Abstract

The invention discloses an environment-friendly degradable packaging bag and a preparation method thereof, belongs to the technical field of packaging products, and adopts a dual compatibilization mechanism of starch surface micro-crosslinking pre-activation and on-line in-situ grafting compatibilization to overcome the defect that corn starch and polyvinyl alcohol molecular chains are both of a strong-polarity polyhydroxy structure; and phase separation is serious due to poor interfacial compatibility. The electromagnetic dynamic plasticizing blow molding technology is adopted to replace traditional resistance heating, and the problem that continuous and stable production is difficult to achieve due to the fact that starch is poor in thermal stability and prone to gelatinization in a melt extrusion method is solved. EMMA is grafted on the interface of starch and PVA in situ to form a polymer brush structure, micro-crosslinked starch constructs a nano-cage rigid framework, water molecule permeation is blocked, and the defect that starch and PVA materials are rich in hydroxyl and prone to moisture absorption and softening is overcome; four-section gradient temperature control pulse heat sealing is adopted, and interface healing is optimized through a molecular chain controllable diffusion-freezing mechanism.

Description

Technical Field

[0001] This invention discloses a packaging product technology, and in particular relates to an environmentally friendly biodegradable packaging bag and its preparation method. Background Technology

[0002] Traditional petroleum-based plastic packaging bags, such as PP and PE, are difficult to degrade in the natural environment, causing serious white pollution.

[0003] While existing biodegradable materials such as polylactic acid (PLA) and polybutylene adipate (PBAT) possess excellent mechanical properties, their high cost limits their large-scale application. Corn starch is widely available, inexpensive, and exhibits excellent biodegradability, but its blending with polyvinyl alcohol (PVA) presents challenges, including poor compatibility, high water sensitivity, and poor water resistance. Current research primarily employs laboratory-scale casting methods to prepare films, resulting in low production efficiency and hindering continuous industrial production. Summary of the Invention

[0004] The purpose of this invention is to provide an environmentally friendly biodegradable packaging bag and its preparation method in order to solve the above-mentioned problems.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing an environmentally friendly and biodegradable packaging bag, comprising the following steps:

[0006] S1, Material pretreatment and micro-crosslinking preactivation: Corn starch is vacuum dried to a moisture content of <1%, and then plasma treated for 3 minutes by a dielectric barrier discharge plasma device. After that, it is sprayed with a 2% solid-liquid mass ratio of 10:1 citric acid solution under microwave conditions at 85℃ for 5 minutes to form micro-crosslinked starch with a crosslinking degree of 0.5-1.5%. The plasma uses argon gas with a flow rate of 20L / min. After that, it is quick-frozen by liquid nitrogen and then crushed into micro-crosslinked starch with an average particle size of 45μm by a high-speed pulverizer.

[0007] S2, reactive extrusion compatibilization grafting: Micro-crosslinked starch, polyvinyl alcohol, sorbitol, and ethylene-methyl methacrylate copolymer (EMMA) are injected into a twin-screw extruder via a side-forced feeding system. The EMMA melt viscosity is controlled at 800-1000 Pa·s. In the reaction section at 195-200℃, 0.03-0.08% peroxide initiator (DCP) is added via a precision metering pump. The EMMA copolymer is then grafted in situ with starch and polyvinyl alcohol to obtain grafted material with a grafting rate of 1.0-2.0%.

[0008] S3, electromagnetic dynamic plasticizing blow molding, feeds the grafted material into the screw extruder, heats it to 180-185℃ by 20kHz magnetic induction, plasticizes it under the action of 0.5-1.0T pulsed electromagnetic field, and blows it into a film through a rotating die head with a blow-up ratio of 4:1;

[0009] S4, gradient temperature control pulse heat sealing: after the film is positioned by vacuum adsorption, it passes through the preheating section, heating section, heat preservation section and pulse cooling section in sequence to complete the heat sealing and obtain the finished packaging bag. The heat sealing cycle is 3 seconds.

[0010] Preferably, in step S2, the twin-screw extruder is equipped with a reaction section in zones four to six, and a two-stage vacuum devolatilization system with vacuum levels of -0.08MPa and -0.095MPa, respectively; the lateral forced feeding system is located at the end of the second section of the extruder, and the EMMA melt is preheated to 185-195°C in the auxiliary extruder.

[0011] Preferably, in step S3, magnetic induction heating and pulsed electromagnetic field are operated by winding a high-frequency induction coil around the outer wall of the screw extruder barrel and installing a pulsed magnetic field generator at the front end of the screw, respectively.

[0012] Preferably, in step S4, a four-segment heat sealing knife is used, with independent temperature control for each segment, and a built-in semiconductor cooling chip and pressure sensor. The process window is set as follows:

[0013] Preheating section: 120℃, pressure 0.2MPa, time 0.5s;

[0014] Heating phase: 180℃, pressure 0.5MPa, time 1.5s;

[0015] Insulation section: 160℃, pressure 0.3MPa, time 0.5s;

[0016] Pulse cooling section: -5℃, pressure 0.1MPa, time 0.5s.

[0017] An environmentally friendly biodegradable packaging bag, prepared by the above method, comprises the following raw materials in parts by weight:

[0018] Surface micro-crosslinked starch: 28-32 parts;

[0019] Polyvinyl alcohol: 28-32 parts;

[0020] Glycerin: 14-16 parts;

[0021] Sorbitol: 4-6 parts;

[0022] EMMA: 2 copies;

[0023] Initiator: 0.05 parts;

[0024] Additives: 1.5 parts.

[0025] Compared with the prior art, the beneficial effects of the present invention are:

[0026] Firstly, the dual compatibilization mechanism of starch surface micro-crosslinking pre-activation and online in-situ grafting compatibilization is adopted to overcome the problem of severe phase separation caused by the poor interfacial compatibility of corn starch and polyvinyl alcohol molecular chains, which are both strongly polar polyhydroxy structures.

[0027] Secondly, the use of electromagnetic dynamic plasticizing blow molding technology to replace traditional resistance heating overcomes the problem of continuous and stable production caused by the poor thermal stability and easy gelatinization of starch in melt extrusion.

[0028] Third, EMMA in situ grafting forms a polymer brush structure at the starch and PVA interface, and micro-crosslinked starch constructs a rigid framework of nanocages to block water molecule penetration, overcoming the disadvantage that starch and PVA materials are prone to moisture absorption and softening due to their high hydroxyl content.

[0029] Fourth, a four-stage gradient temperature-controlled pulse heat sealing method is adopted, which optimizes interface healing through a molecular chain controllable diffusion-freezing mechanism. Attached Figure Description

[0030] Figure 1 The final data charts obtained from the control experiment designed based on the adjuvant system;

[0031] Figure 2 Microscopic morphology of the surface of the packaging bag without EMMA;

[0032] Figure 3 Microscopic morphology of the surface of the packaging bag containing EMMA. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. In this description, it should be understood that the terms "upper," "lower," "front," "rear," "left," "right," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] A method for preparing an environmentally friendly and biodegradable packaging bag includes the following steps:

[0035] S1, Material pretreatment micro-crosslinking pre-activation: Corn starch is vacuum dried to a moisture content of <1%, and then plasma treated for 3 minutes by a dielectric barrier discharge plasma device. After that, it is sprayed with a 2% solid-liquid mass ratio of 10:1 citric acid solution under microwave conditions at 85℃ and reacted for 5 minutes to form micro-crosslinked starch with a crosslinking degree of 1%. The plasma uses argon gas with a flow rate of 20L / min. After that, it is quick-frozen by liquid nitrogen and then crushed into micro-crosslinked starch with an average particle size of 45μm by a high-speed pulverizer.

[0036] The micro-crosslinked structure forms nanocages on the surface of starch granules, preventing starch gelatinization during processing while retaining sufficient hydroxyl groups to react with PVA. A crosslinking degree below 0.5% results in insufficient thermal stability, while a degree above 1.5% reduces reactivity. High-energy electron bombardment at 10-20 eV in argon plasma breaks the C1-C4 glycosidic bonds in starch molecules, generating free radicals at the C2 and C3 positions of glucose.

[0037] Starch-O-CH2-O-CH(OH)-CH(OH)→Starch-O·+·CH(OH)-CH(OH);

[0038] Surface hydroxyl groups are activated by O2 plasma introducing peroxy radicals (-OO·), which oxidize some hydroxyl groups to carboxyl groups (-COOH), increasing the surface charge from -15mV to +28mV (Zeta potential). The original starch granules have a smooth surface, and after plasma treatment, shallow pits of 10-20nm are generated, the roughness Ra is improved, the specific surface area is increased, and more active sites are provided for subsequent cross-linking reactions.

[0039] Microwave-assisted micro-crosslinking of citric acid: molecular behavior and crosslinking reaction pathway.

[0040] Citric acid dehydration: Under rapid heating in a microwave field of 2450MHz, citric acid molecules undergo intermolecular dehydration to form cyclic anhydride intermediates;

[0041] Esterification reaction: The acid anhydride undergoes SN2 nucleophilic substitution with the starch hydroxyl group, forming 70% monoester cross-linking bridges and 30% diester cross-linking rings;

[0042] Starch-OH+HOOC-CH2-C(OH)(COOH)-CH2-COOH→Starch-O-CO-CH2-C(OH)(COOH)-CH2-COOH;

[0043] By adjusting the citric acid concentration to 2% and the microwave power to 500W, the crosslinking density was controlled at 1.0±0.2%, corresponding to 1-1.5 crosslinking points per 100 glucose units;

[0044] Nanocage structure formation: The cross-linking point spacing is about 50-80nm, forming a three-dimensional network space. The molecular chains within the cross-linking domains are confined, and the glass transition temperature Tg increases from 65℃ to 78℃. Key effect: At a processing temperature of 180℃, the cross-linking region maintains the framework structure without collapsing, while the free hydroxyl groups can still react with PVA, achieving the dual functions of rigid support and flexible reaction.

[0045] Low-temperature embrittlement mechanism:

[0046] Liquid nitrogen at -196℃ embrittles the amorphous region of starch, lowering the glass transition temperature (Tg) to -15℃. Under high-speed impact, cracks preferentially propagate along the amorphous region-crystalline region interface, reducing the wafer thickness from 15nm to 8nm and decreasing the crystalline region ratio.

[0047] Criticality of particle size control:

[0048] When pulverized to 45μm, the crystalline-amorphous structure within the particles is preserved, but the size decreases. If the size is too large (>100μm), it leads to uneven dispersion. If the size is too small (<20μm), it damages the crystalline region and reduces the strength.

[0049] S2, reactive extrusion compatibilization grafting: Micro-crosslinked starch, polyvinyl alcohol, sorbitol, and ethylene-methyl methacrylate copolymer (EMMA) are injected into a twin-screw extruder via a side-forced feeding system. The side-feeding temperature is controlled at 190℃, the EMMA melt viscosity is controlled at 800-1000 Pa·s, the main screw speed is 120 r / min, and the residence time is 90-120 s. In the reaction section at 195-200℃, 0.05% peroxide initiator DCP is added via a precision metering pump, reducing the activation energy of the grafting reaction to 85 kJ / mol. Ethylene-methyl methacrylate copolymer (EMMA) is grafted in situ with starch and polyvinyl alcohol to obtain grafted material with a grafting rate of 1.5%.

[0050] In the molten state, DCP decomposes to generate free radicals, which abstract methylene hydrogen from the EMMA molecular chain to form EMMA macromolecular free radicals. These free radicals attack the hydroxyl groups of starch and PVA, achieving in-situ grafting through hydrogen abstraction reactions. The grafting efficiency can reach 1.5%, which is better than the traditional blending efficiency of 0.3%. The grafted chains form a backbone at the interface, increasing the interfacial shear strength from 8.2 MPa to 18.5 MPa.

[0051] The melt flows in layers. The main melt starch / PVA / plasticizer flows at a velocity of 1.2 m / s in the screw channel. The EMMA melt injected laterally cuts in vertically at 190°. The two phases form a tensile flow in the kneading block region, and the interface area is expanded to achieve nanoscale dispersion.

[0052] Shear-induced orientation occurs when the shear rate is high in the kneaded block region, causing the EMMA molecular chains to align along the flow direction. The oriented EMMA chains are more likely to come into contact with starch and PVA hydroxyl groups, thus increasing the reaction probability.

[0053] In step S2, the twin-screw extruder is equipped with a reaction section in zones four to six, and a two-stage vacuum devolatilization system with vacuum levels of -0.08MPa and -0.095MPa, respectively. The first-stage vacuum of -0.08MPa removes unreacted DCP decomposition products such as acetophenone and methane, while the second-stage vacuum of -0.095MPa removes short-chain EMMA oligomers with a molecular weight of <1000.

[0054] S3, electromagnetic dynamic plasticizing blow molding, feeds the grafted material into the screw extruder, heats it to 180°C by 20kHz magnetic induction, plasticizes it under the action of 0.8T pulsed electromagnetic field, and blows it into a film through a rotating die head with a blow-up ratio of 4:1;

[0055] In step S3, magnetic induction heating and pulsed electromagnetic field are achieved by winding a high-frequency induction coil around the outer wall of the screw extruder barrel and installing a pulsed magnetic field generator at the front end of the screw, respectively. The high-frequency induction coil has a frequency of 20kHz and a power density of 50W / cm². 2 The pulse magnetic field generator has a magnetic field strength of 0.8T, a frequency of 50Hz, a barrel temperature of 180℃ (lower than the traditional 210℃), a melt temperature uniformity of ±2℃, and a die head rotation speed of 10rpm.

[0056] Magnetic induction heating achieves volumetric heating with a heating efficiency of up to 95%, avoiding the temperature gradient of traditional resistance heating. The pulsed magnetic field causes the polar molecular chains to generate magneto-orientation, which increases the elastic modulus of the melt and enhances the stability of the film bubble. The electromagnetic ring cooling achieves rapid film bubble shaping through the eddy current effect, increasing the crystallization rate by 50% and refining the spherulite size to 20-30nm.

[0057] An alternating magnetic field of 20kHz generates eddy currents on the screw surface. For stainless steel screws, heat is generated uniformly within the volume rather than through surface conduction. The radial temperature difference of the barrel in traditional resistance heating is ΔT=15-20℃, while in magnetic induction heating: ΔT<3℃, and the melt temperature fluctuates by ±2℃.

[0058] Starch hydroxyl groups -OH and PVA hydroxyl groups are paramagnetic. A pulsed magnetic field of 0.8T and 50Hz causes the hydroxyl groups to orient along the direction of the magnetic field, and the degree of orientation is S≈0.3 as determined by dichroism. Without a magnetic field, the relaxation time of the molecular chain segment τ0=0.05s is shortened to 0.03s under the pulsed magnetic field, the mobility of the molecular chain is improved, and the apparent viscosity decreases from 1200Pa·s to 850Pa·s.

[0059] Moreover, at a shear rate of 100 s⁻¹, the viscosity reduction increased from 65% to 85%, the processing window widened by 30°C, and high-temperature degradation was avoided;

[0060] S4, gradient temperature control pulse heat sealing: after the film is positioned by vacuum adsorption, it passes through the preheating section, heating section, heat preservation section and pulse cooling section in sequence to complete the heat sealing and obtain the finished packaging bag. The heat sealing cycle is 3 seconds.

[0061] In step S4, a four-segment heat sealing knife is used, with independent temperature control for each segment. It incorporates a built-in semiconductor cooling chip and a pressure sensor. The process window settings are as follows:

[0062] Preheating section: 120℃, pressure 0.2MPa, time 0.5s;

[0063] Heating phase: 180℃, pressure 0.5MPa, time 1.5s;

[0064] Insulation section: 160℃, pressure 0.3MPa, time 0.5s;

[0065] Pulse cooling section: -5℃, pressure 0.1MPa, time 0.5s.

[0066] Gradient heating enables the heat-sealing interface to form a controllable melting-diffusion-solidification process, while pulse cooling avoids the deentanglement caused by slow cooling. The heat-sealing interface forms an interpenetrating network (IPN) structure, which improves peel strength. Under this process window, the preheating section is used to eliminate internal stress in the film, the heating section is used to quickly form a molten layer, the heat preservation section is used for molecular chain diffusion and entanglement, and the cooling section is used to quickly freeze the structure.

[0067] Non-contact rapid cooling is achieved using pulsed electromagnetic fields, while the orientation and crystallization behavior of molecular chains are controlled through the magnetostrictive effect, thereby increasing the interfacial strength retention rate; the crystallization process of electromagnetic ring cooling:

[0068] Magnetic field activation period of 0.1s: molecular chain segments are pre-oriented, reducing the nucleation energy barrier;

[0069] Magnetic field shutdown period of 0.1s: rapid cooling, increased nucleation density;

[0070] Repeated cycles: forming fine, uniform microcrystalline structures;

[0071] An environmentally friendly biodegradable packaging bag, prepared using the method described above, comprises the following raw materials in parts by weight:

[0072] Surface micro-crosslinked starch: 30 parts;

[0073] Polyvinyl alcohol: 30 parts;

[0074] Glycerin: 15 parts;

[0075] Sorbitol: 5 parts;

[0076] EMMA: 2 copies;

[0077] Initiator: 0.05 parts;

[0078] Additives: 1.2-1.6 parts.

[0079] While keeping the other raw materials unchanged, the additives usually use 0.5 parts of an antioxidant system and 0.5 parts of a lubricating and dispersing system as essential additives. The main antioxidant is antioxidant 1010, the internal lubricant is calcium stearate 0.2 parts, the external lubricant is ethylene bis-stearamide (EBS) 0.2 parts, and the dispersant is PE wax 0.1 parts. The remaining 0.2 parts of the additives can be selected from heat-stabilizing systems, light-stabilizing systems, and antibacterial and antifungal systems, depending on the material properties.

[0080] Example 1

[0081] In the thermally stable system, the remaining additives are 0.1 parts zinc stearate and 0.1 parts epoxidized soybean oil to capture HCl generated during processing and prevent PVA from being de-HCl degraded.

[0082] Example 2

[0083] In the light-stabilizing system, 0.2 parts of UV-326 are used as other additives to prevent the packaging bags from aging due to light exposure during storage;

[0084] Example 3

[0085] In the antibacterial and antifungal system, the remaining additives are 0.15 parts ε-polylysine and 0.05 parts natamycin, which comply with the GB2760 food additive standard.

[0086] Example 4

[0087] The additives used in the systems of Examples 1-3 were selected comprehensively;

[0088] A control experiment was designed based on the adjuvant system, and the experimental groups are shown in Table 1 below:

[0089] Table 1, Formulation Grouping Details

[0090] ;

[0091] The processing performance indicators are shown in Table 2 below:

[0092] Table 2, Processing Performance Indicators

[0093] ;

[0094] The mechanical and practical performance indicators are shown in Table 3 below:

[0095] Table 3. Mechanical and Practical Performance Indicators

[0096] ;

[0097] Thermal stability performance

[0098] Thermogravimetric analysis (TGA): Nitrogen atmosphere, 10℃ / min, recording the 5% weight loss temperature (T5%) and the maximum degradation temperature (T). max ;

[0099] Oxidation induction period (OIT): 200℃, oxygen atmosphere, determination of oxidation initiation time;

[0100] Continuous processing stability: The rate of change of MFR was measured after 4 hours of continuous production;

[0101] photostable performance

[0102] UV aging test: GB / T16422.3, UVA-340 lamp, irradiance 0.76W / m 2 60℃, aged for 100 hours;

[0103] Performance retention rate after aging: tensile strength retention rate, elongation at break retention rate;

[0104] Yellowing index change Δb: the difference in b value before and after aging;

[0105] Antibacterial and antifungal properties

[0106] Antibacterial rate: GB / T31402, antibacterial rate against Staphylococcus aureus ATCC6538 and Escherichia coli ATCC8739;

[0107] Anti-mildew grade: GB / T24128, Aspergillus niger and Penicillium cordiformis, cultured at 28℃ / 90%RH for 28 days, rated as 0-4;

[0108] Safety: Migration of additives (GB31604.1);

[0109] Degradation and Environmental Performance

[0110] Soil degradation rate: GB / T19277, natural soil, 25℃, burial depth 10cm, weight loss rate in 60 days;

[0111] Safety of degradation products: The impact of degradation solution on seed germination rate;

[0112] The final data obtained is as follows Figure 1 As shown in Groups A and B, the essential additives are indispensable, resulting in a 20.8% reduction in torque; a significant reduction in melt viscosity in the lubrication system, reducing equipment wear and increasing MFR by 71.4%; improved processing fluidity, enabling low-temperature molding and saving 15% in energy; a 31.3% increase in tensile strength; antioxidant 1010 preventing processing degradation and protecting the molecular chains; and a 48° increase in contact angle as the lubricant migrates to the surface to form a hydrophobic layer.

[0113] A 25°C increase in T5% significantly delays the oxidative degradation temperature and widens the processing window; essential additives are indispensable for ensuring process feasibility and basic performance.

[0114] From groups B and C, increasing T5% by 15℃, zinc stearate as an HCl scavenger, and epoxidized soybean oil as an auxiliary plasticizer and stabilizer of the system, the aging retention rate increased by 12%, the residual thermal stress of processing decreased, the long-term stability of the product was enhanced, and the MFR increased slightly by 8.3%; this was due to the weak plasticizing effect of epoxidized soybean oil.

[0115] From groups B and D, the aging retention rate was improved by 16%. UV-326 achieved a molar extinction coefficient of 15000 L / (mol·cm) at 350nm, effectively shielding ultraviolet rays with almost no change in mechanical properties. The amount of UV-326 used was only 0.2 parts, with no dilution effect on the system and a Δb value <3. The yellowing index changed little, and the appearance retention was good.

[0116] From groups B and E, it can be seen that the antibacterial rate reached 99.0%. ε-polylysine adsorbs onto the cell membrane through electrostatic force, disrupting its integrity, while natamycin inhibits fungal ergosterol synthesis. The effect on degradation was weak; ε-polylysine slightly inhibited soil microorganisms, reducing the degradation rate by 2%, which is still within an acceptable range. Migration amount <0.05 mg / dm³. 2 It is far below the limit of 10 mg / dm² in GB31604.1;

[0117] From groups B and F, it can be seen that the processing performance is optimal, with the lowest torque at 34 N·m and the highest MFR at 1.4. The functions of each additive are complementary, resulting in the best overall performance. It has thermal stability, light stability, and antibacterial properties. Zinc stearate not only captures HCl but also acts as a stabilizer for ε-polylysine, preventing its high-temperature degradation. UV-326 is enriched on the surface, while ε-polylysine is enriched in the hydrophilic phase, with clear functional zoning. Epoxidized soybean oil assists in the dispersion of ε-polylysine, preventing its aggregation.

[0118] like Figure 2 and Figure 3The images show the surface microstructure of packaging bags with and without EMMA. The surface of the packaging bag without EMMA is uniform and smooth with a small amount of granular impurities. After adding EMMA, the surface becomes smooth with no obvious particles. This is because EMMA has a certain plasticizing effect. In small amounts, it can insert itself between the corn starch and PVA molecular chains, weakening the interaction forces between the molecular chains and increasing their flexibility. During film formation, the molecular chains can move and arrange more freely, easily forming a smooth surface. EMMA also has a compatibilizing effect; a small amount can improve the compatibility between corn starch and PVA, making the film surface more uniform and smooth. Because EMMA can improve the compatibility between corn starch and PVA to a certain extent, the surface of the packaging bag becomes denser. EMMA molecules interweave between the corn starch and PVA molecular chains, forming an interwoven network structure that is uniformly distributed in the film. As a compatibilizer, EMMA can reduce the interfacial tension between corn starch and PVA, thus making the film more uniform.

[0119] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0120] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing an environmentally friendly biodegradable packaging bag, characterized in that: Includes the following steps: S1, Material pretreatment and micro-crosslinking preactivation: Corn starch is vacuum dried to a moisture content of <1%, and then plasma treated for 3 minutes by a dielectric barrier discharge plasma device. After that, it is sprayed with a 2% solid-liquid mass ratio of 10:1 citric acid solution under microwave conditions at 85℃ for 5 minutes to form micro-crosslinked starch with a crosslinking degree of 0.5-1.5%. The plasma uses argon gas with a flow rate of 20L / min. After that, it is quick-frozen by liquid nitrogen and then crushed into micro-crosslinked starch with an average particle size of 45μm by a high-speed pulverizer. S2, reactive extrusion compatibilization grafting: Micro-crosslinked starch, polyvinyl alcohol, sorbitol, and ethylene-methyl methacrylate copolymer (EMMA) are injected into a twin-screw extruder via a side-forced feeding system. The EMMA melt viscosity is controlled at 800-1000 Pa·s. In the reaction section at 195-200℃, 0.03-0.08% peroxide initiator (DCP) is added via a precision metering pump. The EMMA copolymer is then grafted in situ with starch and polyvinyl alcohol to obtain grafted material with a grafting rate of 1.0-2.0%. S3, electromagnetic dynamic plasticizing blow molding, feeds the grafted material into the screw extruder, heats it to 180-185℃ by 20kHz magnetic induction, plasticizes it under the action of 0.5-1.0T pulsed electromagnetic field, and blows it into a film through a rotating die head with a blow-up ratio of 4:1; S4, gradient temperature control pulse heat sealing: after the film is positioned by vacuum adsorption, it passes through the preheating section, heating section, heat preservation section and pulse cooling section in sequence to complete the heat sealing and obtain the finished packaging bag. The heat sealing cycle is 3 seconds.

2. The method for preparing an environmentally friendly biodegradable packaging bag according to claim 1, characterized in that: In step S2, the twin-screw extruder has a reaction section in zones four to six, equipped with a two-stage vacuum devolatilization system with vacuum levels of -0.08MPa and -0.095MPa, respectively; the lateral forced feeding system is located at the end of the second section of the extruder, and the EMMA melt is preheated to 185-195°C in the auxiliary extruder.

3. The method for preparing an environmentally friendly biodegradable packaging bag according to claim 2, characterized in that: In step S3, magnetic induction heating and pulsed electromagnetic field are operated by winding a high-frequency induction coil around the outer wall of the screw extruder barrel and installing a pulsed magnetic field generator at the front end of the screw, respectively.

4. The method for preparing an environmentally friendly biodegradable packaging bag according to claim 3, characterized in that: In step S4, a four-segment heat sealing knife is used, with independent temperature control for each segment. It incorporates a built-in semiconductor cooling chip and a pressure sensor. The process window settings are as follows: Preheating section: 120℃, pressure 0.2MPa, time 0.5s; Heating phase: 180℃, pressure 0.5MPa, time 1.5s; Insulation section: 160℃, pressure 0.3MPa, time 0.5s; Pulse cooling section: -5℃, pressure 0.1MPa, time 0.5s.

5. An environmentally friendly and biodegradable packaging bag, characterized in that: Prepared by the preparation method according to any one of claims 1-4, comprising the following parts by mass of raw materials: Surface micro-crosslinked starch: 28-32 parts; Polyvinyl alcohol: 28-32 parts; Glycerin: 14-16 parts; Sorbitol: 4-6 parts; EMMA: 2 copies; Initiator: 0.05 parts; Additives: 1.5 parts.

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